UK Self Assessment From Scratch — Part 1: Do You Even Need to File?

Quick answer

Most UK workers never need to do Self Assessment. If you're employed under PAYE and your only income is your salary, your interest stays within the Personal Savings Allowance, and you don't have dividends or rental income, your tax is fully settled through your payslip — no return needed.

You DO need to file if any of these are true for the tax year:

1. You were self-employed with turnover over £1,000.
2. You earned rental income over £1,000 (before expenses).
3. You earned other untaxed income over £1,000 (side hustle, eBay/Vinted reselling, freelancing, crypto).
4. You earned savings interest above your Personal Savings Allowance (£1,000 basic-rate, £500 higher-rate, £0 additional-rate).
5. You earned dividend income above £500 (2024/25 allowance — was £1,000 in 2023/24).
6. You realised capital gains above the £3,000 annual exemption.
7. Your total income exceeded £150,000.
8. You or your partner earned over £60,000 (from April 2024) and one of you received Child Benefit (the High Income Child Benefit Charge, HICBC).
9. You had foreign income to declare.
10. You're a company director with additional income.
11. You want to claim certain reliefs (e.g. higher-rate pension contributions, Gift Aid) and they can't be done via your tax code.

This is Part 1 of 8 in our Self-Assessment From Scratch series — for the full series structure, see the bottom of this post.

The decision flow

A quick mental algorithm:

1. Were you self-employed (sole trader / partnership / CIS) at any point? If yes and gross turnover > £1,000 → file.
2. Did you receive rental income from a UK property? If yes and > £1,000 gross → file (note that the rent-a-room scheme has separate rules up to £7,500).
3. Did you receive any other UK income that wasn't taxed at source? (Side income, gig work, OnlyFans, eBay reselling above hobby level, freelance design, tutoring, etc.) Over £1,000 → file.
4. Did your savings interest exceed your PSA? Most people don't, but with 4% interest rates an additional-rate taxpayer hits zero PSA — file.
5. Did your non-ISA dividends exceed £500? Many investors will trip this in 2024/25 because the allowance shrank.
6. Did you sell any non-ISA shares, crypto, or second property for over £3,000 of profit? → file.
7. Did your total taxable income exceed £150,000? → file (regardless of source).
8. Did anyone in your household receive Child Benefit, and did one of you earn over £60,000? → file for HICBC.

If none of these apply, you most likely don't need Self Assessment.

Edge cases that catch people out

Side hustles you might not think of

The £1,000 trading allowance is a gross number, not profit. You can have business expenses that wipe out the profit and still need to file if turnover exceeded £1,000.

Common ones that creep over £1,000/yr:

• Selling on eBay, Vinted, Etsy at scale. From January 2025, platforms must report seller earnings to HMRC for accounts above 30 sales OR £1,735 (€2,000) per year. Pre-loved clothes for £20 each are fine — bulk reselling isn't.
• Tutoring at £25/hr, even just 2 hrs/week, easily exceeds £1,000.
• Music gigs, paid speaking, occasional freelance writing.
• Renting out a driveway/parking space — often via apps. Over £1,000/yr → file.
• OnlyFans / Patreon / Twitch / YouTube monetisation.
• Crypto staking rewards (treated as income at the time of receipt).
• Cashback from bank accounts of £1+ per month adds up — but most cashback is not taxable (it's classed as a discount on spending, not income). Exception: cashback on business spending.

Untaxed savings interest

Personal Savings Allowance:

At 4.5% interest, £22,000 of cash savings gives £1,000/yr in interest — enough to use up a basic-rate PSA. Above that, the excess is taxable. HMRC usually adjusts your tax code instead of asking for Self Assessment if the amount is modest, but over £10,000/yr of savings income forces Self Assessment.

The £100k–£125,140 trap

If your income is in the £100k–£125,140 band, your personal allowance tapers — and HMRC may struggle to apply this perfectly through PAYE alone. Many earners in this band end up needing Self Assessment to true up the figures, especially with bonuses or salary sacrifice complicating things.

HICBC and Child Benefit

From 6 April 2024, the High Income Child Benefit Charge threshold rose from £50,000 to £60,000, with the full clawback now at £80,000 (was £60,000). This made roughly 700,000 households HICBC-free that previously had to file.

But if either you or your partner earns over £60,000 AND someone in the household claims Child Benefit, the higher earner needs Self Assessment to declare it.

How to register if you do need to file

If you've never filed Self Assessment before, registration is a one-time process:

1. Go to gov.uk/register-for-self-assessment.
2. Pick the right registration route:
   • Self-employed → SA1 form route + becomes a sole trader.
   • Not self-employed but other income → SA1 form alone.
   • Partner in a partnership → SA401.
3. HMRC sends you a UTR (Unique Taxpayer Reference) by post within ~2 weeks. 10 digits. Yours forever.
4. You'll also receive a Government Gateway activation code by post — log in with it to set your online filing account.
5. After that, file at gov.uk/log-in-file-self-assessment-tax-return before 31 January.

Registration deadlines that bite

Late registration penalties are based on the tax you'd have owed — up to 100% if HMRC believes you knew you should have registered. Don't drag your feet on this.

What if you might NOT need to file?

HMRC publishes a "Check if you need to send a Self Assessment tax return" tool at gov.uk. It walks through the questions in this article in interactive form and gives a definitive answer.

If the tool says no but you've previously been told to file, you can also request to stop filing via:

• gov.uk: ask HMRC to remove you from Self Assessment.
• Phone: HMRC Self Assessment helpline (0300 200 3310).

HMRC removes the requirement from the next tax year forward — you'll still need to file for any year you were enrolled.

What's in the rest of this series

This series walks through Self Assessment end-to-end:

• Part 1 — Do you need to file? ← you are here
• Part 2 — Registering, UTR and Government Gateway setup
• Part 3 — Declaring income (employed, self-employed, dividends, foreign, crypto)
• Part 4 — Allowable expenses if you're self-employed
• Part 5 — Capital gains in your Self Assessment
• Part 6 — Payments on account explained
• Part 7 — Making Tax Digital — what's changing in 2026 and beyond
• Part 8 — After you file: refunds, audits, amendments

Parts publish weekly through August and September 2026.

Try the calculators

If your situation involves a side hustle or self-employment:

Or to model how an extra slice of side income changes your take-home:

Sources

• HMRC: Check if you need to send a Self Assessment tax return
• HMRC: Self Assessment tax returns
• HMRC: Register for Self Assessment
• gov.uk: High Income Child Benefit Charge
• gov.uk: Personal Savings Allowance